1. Field of the Invention
The present invention relates to superconductor materials, and, more particularly, to the passivation of high critical transition temperature oxide superconductor materials, such as Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.x.
2. Description of Related Art
Recent developments in high critical temperature (T.sub.c) superconductivity research have led to microwave devices as the first practical application of this new technology. The world's first high performance 6-pole, X-band, bandpass filters, which have applications in low noise amplifiers, have been announced by researchers at Hughes Aircraft's Malibu Research Laboratories.
Such devices employ Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.x (YBCO) films. In order to use such devices in a practical system, the YBCO films must be protected from hostile environments, including air, heat, acids, bases, as well as mechanical abuse such as scrapes, etc. YBCO, whether as high quality thin films or in bulk form, is subject to deterioration when exposed to a large variety of conditions including:
1. Elevation to temperatures above 100.degree. C., which causes loss of oxygen from the YBCO and consequently the degradation of superconductor performance, that is, lower critical transition temperature (T.sub.c) and higher surface resistance (R.sub.s). PA1 2. Corrosion of the YBCO when exposed to acid solutions or vapors; the YBCO will suffer complete degradation. PA1 3. Corrosion of the YBCO when exposed to basic solutions or vapors; the YBCO will suffer complete degradation. PA1 4. Contact with materials which can either react with the YBCO or materials that getter oxygen out of the YBCO material.
Therefore, it is imperative to passivate the YBCO thin films in order to ensure that the device will have long term stable and reliable operation.
Passivation of YBCO superconductors has been reported. Y. Shiraku et al in Japanese Patent Application JP 87-151209 (Jun. 19, 1987) describe the preparation of a passivating layer. MgO powder is mixed with polyimide to form a paste or slurry. This mixture is then spread on YBCO bulk ceramic and fired at between 1,000.degree. and 1,100.degree. C. to form the passivation layer. This kind of heating procedure, however, would completely destroy a thin YBCO film. It is also doubtful that the process described in this reference actually forms a good passivation material, since the polyimide binder is burned off during firing at high temperature and the burn-off of binders usually leads to porosity in the sintered ceramic. Porosity in a passivating film would, of course, be undesirable in a layer designed to encapsulate and passivate the YBCO surface.
Other passivation materials mentioned in this reference and in the scientific literature include SiO.sub.2, Al.sub.2 O.sub.3, AlN, SiN, TiN, Ag, Au, and Bi deposited in an activated oxygen atmosphere. One important deficiency in the literature relating to such materials is the absence of measurements showing the effect of the passivation material on the RF electrical properties of the high T.sub.c superconductor. Measurements of the d.c. electrical resistivity or the low frequency a.c. susceptibility do not necessarily provide a critical way of determining the degradation of the superconductor. Indeed, the present inventors have found that some passivation layers which appear to have good behavior measured by a.c. susceptibility or resistivity, have poor R.sub.s performance.
A need remains for a good passivation coating for YBCO and other oxide superconductors, such as Bi-Sr-Ca-Cu-O and Tl-Ca-Ba-Cu-O.